The invention relates to an electrothermal printer in general. More particularly, the invention relates to a thermal printer provided with an ink ribbon and a heating ribbon independent of the ink ribbon, and with a print head having a plurality of electrodes, the electrodes in the print head and a second electrode being connected across an electrical power supply such as to provide selective heating of the inked ribbon.
Electrothermal printers are disclosed for example in U.S. Pat. Nos. 3,744,611 and 4,456,915. Thermal printers having electrodes incorporated in the print head require the use of complex ink ribbons, provided at least with a layer of high electrical resistance, a layer conductive of electricity and a layer of ink. The manufacturing cost of such ribbons is consequently relatively high, resulting in a high utilization cost for such a thermal printer. Because, in addition, the electrically conductive layer is generally sandwiched between the ink layer and the layer of low electrical conductivity, the heating electrical current is forced to pass twice through the layer of low conductivity. Furthermore, such thermal printers are not adapted to direct printing on thermal sensitive paper.
Another type of thermal printer, described for example in European patent application No. 0086661, is provided with a print head having miniature heating elements, or microresistors, directly heating a simple ink ribbon of low cost. Such print heads are however subjected to considerable heating and are incapable of supplying high power. The printing speed is, consequently, lower than the speed of thermal printers having electrode print heads. In addition, thermal printers of this type have limited application only on smooth paper. The manufacturing cost of microresistors print heads is very high.
Electrothermal printers provided with electrode printing heads are also known, as disclosed in U.S. Pat. No. 4,268,368, which use an ink ribbon which is re-inked after being used once for printing. Such a structure leads to complex and costly printers.